This invention relates to methods for manufacturing electrical temperature sensors, and more particularly to techniques for calibrating such sensors.
Electrical sensors are used to measure temperature in circuits which control a wide variety of equipment, such as building heating, ventilation and air conditioning (HVAC) systems and refrigeration cases. A common type of sensor utilizes a positive temperature coefficient (PTC) silicon detector which has a resistance that varies in relation to the ambient temperature. Thus, the sensors can be connected to a control circuit to provide an input signal which indicates ambient temperature.
Each type of detector has an ideal temperature to resistance response characteristic. However, a particular detector of that type has an actual response characteristic that typically differs from the ideal response. Thus, detectors of the same type will produce slightly different electrical signals when exposed to the same temperature. Furthermore, it often is desirable to optimize the response characteristic for a range of temperatures, for example a range which enables the device to be used in either a refrigeration case or a building room. The detector response characteristics also may have to be adjusted to be compatible with the control circuits.
As a consequence, in order to accurately measure temperature, it is necessary to calibrate the detector, that is change the actual response of the detector to match the ideal desired response as closely a practical. Calibration typically is accomplished by connecting trimmer resistors to the detector to form the complete temperature sensor. Specifically, one trimmer resistor is placed in series with the detector to offset the actual temperature response to the level of the ideal desired response. Another trimmer resistor is placed in parallel with the temperature detector to change the gain of the sensor (i.e. adjust the slope of the temperature response characteristic) to match the ideal desired response.
Previous calibration techniques required a significant amount of manual labor. Specifically, individual detectors were placed into a precision oven and heated to two different temperatures near the extremes of the desired temperature range for the completed sensor. The electrical signal produced by the sensor at each temperature was measured. These measurements then were fed into a computer which calculated values for the two trimmer resistors. Assembly workers then selected the appropriate valued resistors which were hand soldered to the detector in order to produce a finished calibrated temperature sensor. This process not only was labor intensive, but time consuming.